This invention relates generally to aerodynamic surfaces and, more particularly, to improved constructions for such aerodynamic surfaces which provide for aerodynamic control.
In general, the aerodynamic efficiency of any lifting surface, regardless of the type of vehicle, is dependent on the lift-to-drag ratio of that surface. Numerous schemes are known for controlling aerodynamic surfaces on rotor blades, wings, engine inlets, fan blades, and nozzles. Movable control surfaces placed on these aerodynamic surfaces have included flaps, slats, spoilers, ailerons, elevators, and rudders. Although these control surfaces can mechanically alter the geometry of the original aerodynamic device, they are limited in their ability to respond quickly and efficiently. Furthermore, such mechanical control surfaces may have a number of disadvantages, including added complexity to the aircraft, reduced structural integrity, complicated manufacturing, and compromised radar detectability.
In an attempt to mitigate some of these disadvantages, McDonnell Douglas, now part of Boeing, has successfully incorporated devices known as synthetic jet actuators into various aerodynamic surfaces, for example, helicopter blades. A synthetic jet includes a movable diaphragm positioned within a chamber. Movement of the diaphragm pulses air in and out of the chamber through an orifice. In the context of a wing or blade, the diaphragm is positioned within a hollow portion of the structure and pulses air in and out of one or more orifices in the outer skin. The outer skin may be made xe2x80x9cporousxe2x80x9d with the wing or blade having a plurality of synthetic jets incorporated therein. See, for example, U.S. Pat. Nos. 5,813,625 and 5,938,404.
In the ""404 patent, a voice coil mechanism is used to displace a piston attached around its periphery by a diaphragm. The diaphragm and piston define a hollow chamber within the aerodynamic structure, the space being open through a number of orifices in the skin of the structure. The piston is driven at relatively low frequencies compared to piezoelectric diaphragms. This type of device can be termed a first generation Boeing synthetic jet actuator. Despite providing improvements over mechanically-displaced surfaces, the effectiveness of such devices is limited above relatively low operational frequencies. For example, such synthetic jet actuators suffer performance degradation at oscillation frequencies above 200 Hz, which is at the lower end of the range of operational frequencies in a free stream environment of moderate to high subsonic Mach numbers (e.g., 0.20-0.50).
There is presently a need for an improved system for controlling aerodynamic surfaces at moderate to high subsonic free stream Mach numbers (e.g., 0.20-0.50) that reduces manufacturing complexity and does not compromise radar detectability.
The present invention provides an improvement to existing electromagnetic synthetic jet actuators that suffer from performance degradation in the form of low-momentum output at high operational frequencies (typically about 200 Hz). The present invention thus enables realization of aerodynamic benefits at moderate to high subsonic free stream Mach numbers (e.g., 0.20-0.50), such as enhancement of aerodynamic lift force and/or reduction of vehicle drag. Moreover, these aerodynamic benefits are realized consistently over a wide range of frequencies. In general, the present jet actuators minimize inertial loads associated with moving components, provide a stiffer piston for compressing a flexible diaphragm so as to minimize elastic losses, and facilitate the ingestion of ambient fluid (e.g., air) by the actuator during the suction portion of the oscillation cycle.
In one embodiment, the present invention provides a jet actuator for control of an aerodynamic structure, the structure including a hollow space adjacent an aerodynamic surface and an orifice opening through the aerodynamic surface adjacent to the hollow space. A substantially rigid movable member fits within the hollow space and includes a pair of pistons connected by an elongated cross element defining an axis. The movable member is substantially symmetric in terms of its mass about a plane extending perpendicularly through the mid-point of the cross element, and the axis extends in a direction that intersects the aerodynamic surface substantially normally so that one of the pistons is an outer piston and the other is an inner piston. Each of a pair of springs couples one of the pistons to one or more fixed points relative to the aerodynamic structure so as to define a point of equilibrium for the movable member along the axis. A bellows within the hollow space is sealed around the orifice to define a compression chamber open to the exterior of the aerodynamic surface, the outer piston being attached to. the bellows. A voice coil mechanism within the hollow space drives the movable member in both directions from its point of equilibrium along the axis and causes the outer piston to alternately compress and expand the bellows, thus expelling fluid from and pulling fluid into the compression chamber through the orifice.
In a preferred embodiment, the pistons are flat and thin relative to the axis so that the movable member is in the shape of a spool. The pistons may be circular or other non-circular shape such as rectangular. The movable member is desirably made substantially of composite materials. For example, the outer piston includes at least one, and preferably multiple layers of uni-directional graphite laminated with a composite honeycomb structure. The springs may comprise a pliable membrane extending between the periphery of each of the pistons and a fixed location within the hollow space. The voice coil mechanism preferably includes a pair of spaced apart magnets each having a through opening, with the cross element extending through the openings and the pistons being located on oppositely-facing sides of the magnets. Each of a pair of electric coils rigidly attaches to one of the pistons facing the adjacent magnets and extends within a cavity in the magnets for movement therewithin. An electric circuit energizes the electric coils to drive the pistons in tandem.
In another embodiment of the present invention, a jet actuator for control of an aerodynamic structure is provided, the aerodynamic structure having an aerodynamic surface and hollow space therewithin. An orifice opens through the aerodynamic surface adjacent the hollow space and a substantially rigid movable member having a piston fits within the hollow space and moves along an axis. A flexible diaphragm provided within the hollow space seals around the orifice to define a compression chamber open to the exterior of the aerodynamic surface. The piston is attached to the flexible diaphragm such that movement of the piston along the axis alternately compresses and expands the chamber. A voice coil mechanism within the hollow space drives the movable member along the axis and causes the piston to alternately compress and expand the compression chamber, thus expelling fluid from and pulling fluid into the chamber through the orifice. A one-way valve opening to the compression chamber permits fluid to be pulled therethrough into the chamber upon. expansion of the chamber, but prevents fluid from being expelled therethrough from the chamber upon compression of the chamber.
In a preferred embodiment, the one-way valve is located in the aerodynamic surface, and preferably a plurality of such valves surround the orifice. Alternatively, the one-way valve is located in the piston. The one-way valve may comprise a flapper valve, such as a plate-like structure anchored along one edge and cantilevered over an opening to the compression chamber. In one embodiment, the plate-like structure is made of a composite material, while in another embodiment the material is stainless-steel. Desirably, each of a plurality of plate-like structures is anchored within a recess formed in an inner face of the aerodynamic surface.